1. Field of the Invention
Removal of harmful nitrogen compounds such as nitrites and nitrates from aqueous solutions and denitrification of flue gases and waste gases from thermal power installations by the wet-absorption process (scrubbing of nitrite and nitrate solutions).
The invention relates to the further treatment and processing of waters from denitrification and potable water treatment units, which are unsuitable for direct use or discharge into the effluent and have an inadmissibly high content of nitrogen salts. The effective removal of nitrites and nitrates is gaining increasing importance with regard to the question of solving environmental problems.
In particular, it relates to a process for complete removal of nitrites and nitrates from an aqueous solution by means of electrolysis.
2. Discussion of Background
In conjunction with the endeavors to improve the environment, to reduce air pollution and to protect surface waters, the removal and degradation of harmful nitrogen compounds are gaining importance. This applies in particular to the treatment and further processing of waters containing nitrites and nitrates. In the removal of nitric oxides from combustion waste gases by aqueous absorption or of nitrate from potable water by membrane processes, aqueous salt solutions are formed which contain nitrite and/or nitrate in a relatively high concentration. The arising of such solutions containing nitrite and/or nitrate calls increasingly for suitable solutions of the associated problems.
In the current state of the art, the wet absorption of nitric oxides from combustion waste gases is carried out by first oxidizing the NO to NO.sub.2 in the gas phase by means of an oxidizing agent such as ozone, followed by absorption in an alkaline scrubbing solution. In this case, the absorbents must be fed continuously and the nitrate-containing scrubbing solution must be discharged.
It is known from the literature that, in the electrolysis of aqueous solutions containing nitrite and/or nitrate, NO, N.sub.2 O and NH.sub.3 are also formed in addition to hydrogen at the cathode. The composition of the cathode gas depends on the process conditions. It is also known that nitric oxides NO.sub.x can be catalytically converted by means of NH.sub.3 or H.sub.2 to N.sub.2 and H.sub.2 O and can thus be rendered harmless.
Regarding the state of the art, the following literature references are cited:
G. Kreysa, G. Breidenbach, K. J. Muller, Untersuchung zum Mechanismus und zur Katalyse der elektrochemischen Salpetersaure-reduktion [Study of the mechanism and catalysis in the electrochemical reduction of nitric acid], Ber. Bunsenges. Phys. Chem. 87, 66-71 (1983). PA1 K. W. Lexow, Elektrolytische Reduktion von NaNO.sub.3 in NaOH mit Raney-Nickel-Elektroden [Electrolytic reduction of NaNO.sub.3 in NaOH with Raney nickel electrodes], Dechema Monographs 98, 229-244 (1985). PA1 M. Schrod, J. Semel, R. Steiner, Verfahren zur Minderung von NO.sub.x -Emissionen in Rauchgasen [Processes for reducing NO.sub.x emissions in flue gases], Chem.-Ing.-Tech. 57 (1985) No. 9, pages 717-727.
The known processes by themselves no longer satisfy the current requirements. There is therefore a demand for further developing and perfecting them.